Adding support of more general int-like objects in islice() looks reasonable to me. I'm not sure about permutations(), but if make this change, use PyIndex_Check() instead of PyNumber_Check(), or don't use the special check at all, allowing PyNumber_AsSsize_t() to fail.

The __setstate__() methods shouldn't be changed. We know that the __reduce__() methods return exact ints, not general int-like objects.

When replace PyLong_AsSsize_t with PyNumber_AsSsize_t take into account that PyLong_AsSsize_t is atomic and thread-safe, while PyNumber_AsSsize_t can call Python code and release the GIL.